Conventionally, as an active material for a negative electrode of an electrochemical element with a nonaqueous electrolyte, an electrode material such as a layered compound, for example, graphite in which lithium can be removably inserted, or a metal or metal oxide that can be alloyed with lithium has been used, for example, in the case of lithium secondary batteries. Graphite-based negative electrode material has excellent reversibility with respect to an electrochemical oxidation-reduction cycle. This is because only the occlusion or discharge of lithium between layers of the layered compound occurs during charging and discharging, so that the crystal structure of the electrode material itself does not change considerably. However, the above-mentioned graphite-based negative electrode material utilizes the back and forth movement of lithium between the layers, and thus, at most one lithium atom reacts with six carbon atoms. In theory, it is difficult to achieve a capacity of 372 mAh/g or more using such a method.
On the other hand, other negative electrode material that can be alloyed with lithium can achieve a large discharge capacity that exceeds 1000 mAh/g. One example is silicon where three or more lithium atoms react with one silicon atom. However, there is a problem that such electrode material particles have a high swelling/contracting rate with the alloying of lithium, which deteriorates their cycle characteristics.
Accordingly, as a negative electrode material that has excellent cycle characteristics and is capable of achieving a higher capacity, a lithium transition metal composite nitride has been developed. This composite is expressed by a general formula LijMkNm wherein M is a transition metallic element, j>0, k>0, and m>0 (see M. Nishijima et. al., Solid State Ionics, vol. 83, 107(1996)). This electrode material is a layered compound similar to the graphite and exhibits an excellent reversibility with respect to an electrochemical oxidation-reduction cycle. Further, it also achieves a high capacity of 800 mAh/g or more, and has been attracting attention.
However, such a lithium transition metal composite nitride has the following problem. It is very unstable in the air, reacting with moisture therein, and decomposing into LiOH or Li2CO3 leading to loss of function as an electrode material. Therefore, when using this composite as an electrode material, it has to be dealt with in a dry room or the like, which may complicate the manufacturing process.
In relation to this, JP 2001-15101 A discloses a method of coating at least a part of a surface of such a lithium transition metal composite nitride with an electrically conductive material using a mechanofusion technique. However, this method attempts to give the lithium transition metal composite nitride an electrical conductivity and does not consider stability in the air.
According to the inventors' research, it was found that the above-described method using the mechanofusion technique cannot give the surface of the lithium transition metal composite nitride sufficient stability in the air because the surface tends to be coated ununiformly. Furthermore, when a thick coating is formed so as to give a certain stability in the air, the charge-discharge reaction of the lithium transition metal composite nitride is inhibited.
The present invention was made in order to solve the conventional problems described above, and an object of the present invention is to provide a composite electrode material with improved stability in the air as an electrode material capable of constituting an electrochemical element with increased capacity and improved cycle characteristics.